1. Field of the Invention
This invention relates to the relative positioning of independent seismic arrays with respect to a central station by use of relative differences of electromagnetic positioning signals.
2. Discussion of the Prior Art
In modern seismic exploration practice on land, it is customary to employ several hundred to a thousand or more remote seismic sensor arrays, deployed in a predetermined relationship within a desired region, usually within a few kilometers, relative to a central station. Centralized hard-wired systems and radio-controlled systems are known. But in a preferred system, the remote units have no physical or ethereal connection with the central station during a normal recording period. They are completely incommunicado with respect to the central station. For example, see U.S. Pat. No. 4,281,403 assigned to a sister firm of the assignee of this invention. In that patent, a plurality of remote recording units, each having a sensor array coupled thereto are provided. Each remote unit has an internal alarm clock that wakes up the unit for a period of time at programmed intervals, according to the local clock so that the unit can record seismic signals generated by a sound source that is fired at corresponding intervals by the central station according to a master clock.
In the above system, two things are necessary: Each remote unit must have a unique identity and the geographic location of each unit must be known.
Unit identification is simple. A unique number is simply hard-wired into each unit or is entered via a BCD thumbwheel switch. When data are recorded on a cassette tape or memory that is included in each unit, the identification number is recorded in the header of each data record.
Normally a land surveyor stakes out the precise locations that the central station and the remote recording units are to occupy. Each location is identified according to some code that is related to the geographic coordinates of the location. Customarily, field attendants manually write a comparative table of unit identification versus location code. Later, after the recorded data have been collected from the remote sensors and processed, the processed data can be related to specific surveyed locations on the earth's surface.
Where many hundreds of remote sensors are involved at each setup, manual documentation becomes a formidable task indeed. Therefore the opportunity for gross tabulation errors is unacceptably high.
It is an object of this invention to automatically acquire and record electromagnetic position measurement signals from which can be determined the approximate location of each of a plurality of remote seismic recording units with respect to a central station. It is not intended that the location information calculated by this invention be a substitute for precise land surveying. It is intended that the accuracy of the method will be just sufficient to associate each uniquely identified remote seismic recording unit with that particular presurveyed location at which the unit is positioned.